Add a couple error case tests too

Author: tianon

Dockerfile.test

@@ -64,4 +64,8 @@ RUN [ "$(gosu 1000:root env | grep '^HOME=')" = 'HOME=/' ]
 RUN [ "$(gosu games env | grep '^HOME=')" = 'HOME=/usr/games' ]
 RUN [ "$(gosu games:daemon env | grep '^HOME=')" = 'HOME=/usr/games' ]
 
+# make sure we error out properly in unexpected cases like an invalid username
+RUN ! gosu bogus true
+RUN ! gosu 0day true
+
 # something missing?  some other functionality we could test easily?  PR! :D